The invention relates to a method for evaluating myocardial blush in tissue from images recorded following injection of fluorescent dyes.
TIMI (Thrombolysis In Myocardial Infarction) studies initially suggested that successful restoration of flow in an infarcted artery was the major goal of reperfusion. However, substantial evidence has grown over the years showing that distortion of microvasculature and myocardial perfusion is often present despite epicardial artery patency. This might be the result of a combination of distal embolization and reperfusion injury with cellular and extracellular edema, neutrophil accumulation and release of detrimental oxygen free radicals.
Myocardial blush was first defined by van't Hof et al. as a qualitative visual assessment of the amount of contrast medium filling a region supplied by an epicardial coronary artery. It is graded as Myocardial Blush Grade: 0 (=no myocardial blush or contrast density), 1 (=minimal myocardial blush or contrast density), 2 (=myocardial blush or contrast density which exists to lesser extent and its clearance is diminished compared to non-infarct-related coronary artery), and 3 (=normal myocardial blush or contrast density comparable with that obtained during angiography of a contralateral or ipsilateral non-infarct-related coronary artery). When myocardial blush persists (long “wash-out rate” or “staining”), it suggests leakage of the contrast medium into the extravascular space or impaired venous clearance and is graded 0.
The consequences of microvascular damage are extremely serious. In patients treated with thrombolytics for acute myocardial infarction, impaired myocardial perfusion as measured by the myocardial blush score corresponds to a higher mortality, independent of epicardial flow. Myocardial blush grade correlates significantly with ST segment resolution on ECGs, enzymatic infarct size, LVEF, and is an independent predictor of long-term mortality. Myocardial blush grade may be the best invasive predictor of follow-up left ventricular function. Determining the myocardial blush has emerged as a valuable tool for assessing coronary microvasculature and myocardial perfusion in patients undergoing coronary angiography and angioplasty.
The degree of blush that appears during imaging (e.g., imaging with a fluorescent dye, such as ICG) is directly related to the underlying tissue perfusion. Conventionally, to quantitatively characterize kinetics of dye entering the myocardium using the angiogram, digital subtraction angiography (DSA) has been utilized to estimate the rate of brightness (gray/sec) and the rate of growth of blush (cm/sec). DSA is performed at end diastole by aligning cine frame images before the dye fills the myocardium with those at the peak of a myocardial filling to subtract spine, ribs, diaphragm, and epicardial artery. A representative region of myocardium is sampled that is free of overlap by epicardial arterial branches to determine the increase in the grayscale brightness of the myocardium at peak intensity. The circumference of the myocardial blush is then measured using a handheld planimeter. The number of frames required for the myocardium to reach peak brightness is converted into time by dividing the frame count by the frame rate. This approach is quite time-consuming and is difficult to perform on a beating heart and to conclude within a reasonable time.
Generally, conventional techniques gathering statistical information about a ROI rely on algorithms that track the ROI during movement of the underlying anatomy and attempt to keep the ROI localized in the same tissue portion. For example, the user can draw an initial ROI in the image, ignoring any blood vessels not to be included in the calculation, with the initial ROI then adjusted to the moving anatomy through linear translation, rotation, and distortion. However, this approach is computationally intensive and not reliable with low contrast images.
Accordingly, there is a need for a method to determine blush of myocardial tissue while the heart is beating, to eliminate effects from features other than myocardial tissue that may migrate into the region of interest (blood vessels, clips, the surgeon's hands, etc. . . . ), and to produce useful information for the surgeon during a medical procedure within a “reasonable time,” if not within “real time.”
There is also a need for measuring improvement in cardiac function by measuring the time differential between when contrast in a blood vessel reaches its peak intensity and when the contrast in a neighboring region in the myocardial tissue reaches its corresponding peak. If this time differential decreases after a medical procedure as compared to before the procedure, under uniform hemodynamic conditions cardiac function can be said to have improved. A method for tracking blood vessels during image acquisition improves our ability to locate the time at which the contrast in a blood vessel achieves its peak intensity.